Executive control is the most important of the cognitive abilities developed in childhood. It includes the ability to selectively attend to relevat information, allocate limited attentional resources appropriately, switch between simultaneous tasks, and manipulate information held temporarily in mind. Executive control is one of the strongest predictors of academic achievement and long-term well being. Previous research has shown that bilingual children develop executive control earlier and perform executive control tasks more efficiently than their monolingual peers. The present proposal extends this research by investigating the mechanism by which bilingualism modifies these abilities and determining the limits of bilingualism in modifying this development for children who have executive control challenges. The two challenges to executive control investigated here are poverty and attention-deficit hyperactivity disorder (ADHD), both situations that lead to delayed or impaired development. The research will use both cross-sectional and longitudinal designs to study these processes as bilingualism is emerging, both behavioral and electrophysiological (ERP) methods to document possible changes in brain response that are associated with this cognitive development, random assignment to groups in a training study to establish causality, and both typically-developing and challenged populations (poverty, ADHD) to investigate these developments in children at risk for poor executive control. Although there is substantial evidence for precocious development of executive control in bilingual children and delayed development of executive control in children in poverty and with ADHD, there is no research to date that investigates whether bilingualism continues to confer positive effects on development for children in these challenging situations. The research will reveal how neuroplasticity operates in children's development and how bilingualism in particular modifies cognitive and brain function. Understanding how a specific experience enhances the development of executive control has clear translational impact for creating interventions, remediation approaches, and instructional strategies to maximize the development of this crucial ability for all children.